Acetyl CoA carboxylase activity in rat liver homogenates show heat-activation in a time- and temperature-dependent process. The ability to heat-activate has been shown to be hormone and dietary dependent. Liver homogenates have been fractionated to separate a heat stable inhibitor of the heat activation reaction, tentatively identified as oxidized glutathione, and an activator of the reaction, sulfate. Enzymes involved in the activation and inactivation processes will be purified and their relationship to citrate and fatty acyl CoA control will be studied.